A typical loudspeaker assembly for low to mid-range audible sound reproduction, that is, a bass speaker, or a “woofer”, has a magnetic sandwich assembly. The core of the sandwich is typically, an annular magnet, although square or rectangular open center magnets are also known. A backplate, typically circular, is mounted to one annular face of the magnet, and has an outside diameter similar to, the outside diameter of the annular magnet. An annular top plate is mounted to the other face of the magnet. The top plate has an outside diameter similar to the backplate, and an inside diameter less than the inner diameter of the annular magnet. The magnet, the backplate, and the top plate are concentrically mounted about a central axis perpendicular to the annular magnet. The polarity of the magnet is such that the annular top and bottom faces are opposite poles.
In general, the magnet sandwich assembly has a central post, sometimes called a pole piece, a T-yoke, or a core. The pole piece is mounted along the central axis of the magnetic assembly to extend from the backplate toward the top plate. The end of the pole-piece standing furthest away from the backplate usually sits adjacent the inner annular edge or face of the top plate. A small radial gap is left between the top plate and the end of the pole piece. Inasmuch as the top plate, the backplate, and the pole-piece are typically made of ferromagnetic materials, or materials of high magnetic permeability, a magnetic flux field is established by the magnet across the small gap. Thus the magnetic flux path lies completely in materials of relatively high magnetic permeability except for the gas, typically air, in the gap between the inner edge of the annular member and the central post.
A voice coil is suspended to ride in the small gap, which is the location of maximum magnetic flux density. The voice coil can reciprocate linearly relative to the pole piece, and, in so doing, the turns of the coil traverse the field of the magnet. The voice coil is suspended from a resilient suspension, usually in the form of a moving diaphragm assembly mounted on springs. The diaphragm assembly usually has a membrane element that has the shape of a truncated section of a cone, with a dust cap membrane extending across a narrower part of the section. The lead wires for the coil are usually, though not always, mounted to the cone, typically with glue, and the cone and the voice coil are constrained to move together.
Loudspeaker performance reflects design compromises in the desired power output, the choice of magnet, the choice of coil, the nature of the resilient mounting, the entrapment of air in the assembly, and the size and shape of the cabinetry placed about the loudspeaker itself. A loudspeaker will tend to have a chosen desired operating bandwidth. In general, when operated outside the design bandwidth the speaker is more likely to exhibit non-linear performance, and more likely to exhibit undesirable, or less than optimal, impedance characteristics, and may tend to be more prone to failure.
In operation, the varying electrical current in the voice coil will tend to cause the voice coil to become warm. The heating of the coil is known, and one design consideration is the anticipated operating temperature. When the coil becomes warm, the adjacent metal structure also becomes warm, though generally to a lesser degree. The electrical resistance of copper wire increases with increasing temperature. Inasmuch as the power output of a speaker varies inversely as the resistance of the voice coil, an increase in the temperature of the coil will tend to reduce the current in the coil, and reduce the output power. As a consequence, it is advantageous to keep the voice coil relatively cool, if possible. If the coil is operated too vigorously, its performance will deteriorate, and may ultimately fail. Voice coils of aluminium wire are less likely then copper coils to have a general rise in impedance with increasing power, but also have a tendency to fail with increasing temperature.
In the past, a common solution to the problem of increased resistance due to coil warming has been to choose a larger, heavier speaker to give a higher output power level. A larger voice coil, magnetic assembly, or both, will usually be capable of producing the same power output as a smaller one but with a lower temperature rise, and less deterioration in performance. To maintain the same specified ratio of output to losses, Qes, and therefore Qts, it would be customary to increase both the size of the magnet and the size of the coil. It would be advantageous to reduce the operating temperature of the voice coil to permit a smaller motor, {that is, a coil and magnet assembly), to be used at higher power levels, rather than having to adopt a bigger, heavier, and possibly more expensive, unit.
At present, it is not uncommon for the pole piece of a woofer to be hollow, and for the back plate to have a central opening. When the diaphragm moves, the air pocket beneath the dust cover membrane then has an outlet to ambient through the hollow pole piece and through the backplate, so pressure variation in the pocket is reduced. One attempt to use the motion of the air displaced by the dust cover dome to enhance cooling is shown in U.S. Pat. No. 5,042,072 of Button, issued Aug. 20, 1991. Button forms three peripheral grooves in the outer face of the pole piece. These channels vent through the backplate. The usually open hollow core of the pole-piece is blocked such that air is forced to move in or out through the three channels. Inasmuch as the radially outer portion of the channels is bounded by the inner face of the voice coil, Button indicates that portions of the voice coil are cooled by forcing air displaced by the dome through the channels next to the voice coil.
U.S. Pat. No. 5,357,586 of Nordschow and Wright, issued Oct. 18, 1994. It shows a speaker that has radial perforations in the voice coil former at a level above the voice coil. A vane, or aerodynamically shaped body, is mounted within the hollow core of the pole-piece. Air flows more easily past the aerodynamically shaped body in one direction than the other, such that reciprocation of the diaphragm will tend to cause a flow of air through the assembly between the perforations and the opening in the backplate, with a cooling effect.
U.S. Pat. No. 5,497,428 of Rojas issued Mar. 5, 1996. It shows a loudspeaker assembly with channels formed in the external periphery of the pole-piece adjacent to the voice coil. These channels vent into the hollow central core of the pole piece, which, as is customary, is open through the backplate to ambient. The top end of the pole piece is blocked by a generally conical part, such that the air is forced to flow through the channels.
Another noted phenomenon of existing loudspeakers in which the backplate of the speaker is a closed plate, is that the reciprocation of the diaphragm assembly, as in a closed back mid-range to high audible frequency unit, or tweeter, will tend to compress or expand, the air trapped within the loudspeaker casing itself. Where the entrapped volume is small, the effect can be quite pronounced. It is desirable in such instances to increase the internal trapped volume so that the volume displaced by operation of the speaker diaphragm is small relative to the trapped volume, with a consequent lessening of the pressure fluctuations in the entrapped gas. It will be apparent that compression of the entrapped gas will tend to oppose the motion of the speaker, and may complicate it still more if there is a dynamic resonance problem. A known method of reducing the magnitude of this phenomenon is to provide a larger backshell or housing. This is generally not desirable because it increases the size of the loudspeaker unit for the purpose of enclosing air. It would be advantageous to increase the volume of the entrapped air without having to increase the physical size of the loudspeaker envelope.
U.S. Pat. No. 5,335,287 of Athanas, issued Aug. 2, 1994 relates to a loudspeaker employing a magnetic liquid suspension for locating the voice coil in the gap. According to Athanas, one problem of magnetic liquid voice coils is that the liquid has a tendency to be blown, or drawn, out of the magnetic gap. In Athanas' view this was because the oscillatory motion of the voice coil produces momentary changes in the atmosphere near the end of the pole piece, and in the annular chamber surrounding the pole piece. The loudspeaker shown by Athanas has not only a port through the back plate to vent the hollow core of the pole piece, but also additional vents formed through the backplate to vent the annular chamber formed between the pole piece and the magnet. This is thought to reduce the tendency of the air compressed in the annular chamber from pushing the liquid out of the gap.